1. Field of the Invention
A compact linear accelerator (linac) which produces high electron beam brightness by accelerating a tightly focused electron beam generated from a laser illuminated photocathode in an integrated, multi-cell, X-band rf linac.
2. Description of the Prior Art
Integrated photoelectron linear accelerators have been available in the prior art. For example, the inventors of the apparatus disclosed herein have previously developed a S-band integrated photoelectron linac focused by a set of compact solenoids to provide the necessary magnetic field for emittance compensation. The S-band linac employs a plane wave transformer (PWT) design which has advantages over conventional cup-and-washer linac design. The S-band integrated PWT photoelectron linac has been installed at a local university (UCLA) and utilizes a 20-MW, S-band klystron with a pulse length of 2.5 xcexcsec and a repetition rate of 5 Hz as the rf power source, a Nd:YLF laser for the photocathode and a cooler/pressure control for the thermal/flow control of the PWT to produce a bright electron beam.
The PWT design of a linac structure was first referenced several decades ago by and was subsequently incorporated, though uncommonly, in several devices. For example, U.S. Pat. No. 5,014,014 to Swenson discloses a plane wave transformer linear accelerator for accelerating charged particles to high velocities and incorporates a tank section having end plates and iris-loaded washers supported by rods extending between the end plates. While the first Swenson linear accelerator built for UCLA never operated, a second PWT linac, again not integrated to the photocathode, built by UCLA did perform quite well. However, it has a serious disadvantage in that the linac is separated from the photocathode by a long drift section. As a result, the low-energy electron beam from the first short photoinjector gets a strong kick at the exit of the photoinjector and its emittance is degraded at the entrance to the drift tube. Complex external rf and magnetic subsystems are required in order to operate this photoelectron linac. In addition, this earlier UCLA PWT linac does not provide electrons of sufficient brightness for some commercial and high energy physics application. Although the new S-band integrated linac mentioned hereinabove does provide excellent results, many applications require a still brighter electron beam than can be produced thereby.
What is desired is to provide an integrated PWT photoelectron linac that provides an extremely bright electron beam required in research, industry and medicine.
The invention provides a compact high energy X-band photoelectron injector which integrates the photocathode directly into a multi-cell linear accelerator with no drift space between the injector and the linac. By focusing the beam with permanent magnets, and producing high current with low emittance, extremely high brightness is achieved. In addition to providing a small footprint and improved beam quality in an integrated structure, the compact system simplifies external subsystems required to operate the photoelectron linac, including rf power transport, beam focusing, vacuum and cooling. The photoelectron linac employs a Plane-Wave-Transformer (PWT) design which provides strong cell-to-cell coupling, relaxes manufacturing tolerance and facilitates the attachment of external ports to the compact structure with minimal field interference. An enhanced brightness, X-band integrated photoinjector using a PWT and producing electron energy of tens of MeVs in a much smaller footprint, important for many commercial applications, is thus provided by the present invention.
The X-band PWT photoelectron linac of the present invention produces high-charge, relativistic electron bunches with subpicosecond duration. This, combined with high beam quality and extremely low emittance, will result in vastly increased beam brightness. A wide range of potential beneficiaries of a high-brightness electron beam includes future linear colliders, new-generation synchrotron radiation rings, and other electron beam based light sources such as free electron lasers (FEL) and Compton backscattering X-ray sources, as well as many applications further discussed below.
Compact, high-brightness, electron accelerators have many uses. They are widely used in nearly every field of physics from elementary particles to solid-state materials. They are also essential instruments in many fields of research for the study of structures in chemistry and biology, or for sensitive trace-element analysis. Compact linacs are useful in the fields of health, food preservation, energy, environmental monitoring and protection, and industrial processing. RF linacs can be used at low energies (several tens or hundreds of MeV) as injectors into synchrotrons and FELs or at high energies as particle colliders, accelerating electrons and positrons to hundreds or thousands of GeV. Accelerators have probably found their widest field of application in medicine, such as in tracer isotope production for nuclear medicine, or in X-rays, gamma, or charged particle production for diagnostics or therapy. In fact, compact rf electron linacs have been installed in over a thousand hospitals worldwide.
The present invention can be used with synchrotron radiation facilities as an injector into small emittance advanced storage rings, or to produce short wavelength coherent radiation using FEL interaction. In addition the proposed system can be used together with a terawatt, table-top laser to produce nearly monochromatic X-rays by Compton backscattering, of intensity comparable to that of second generation synchrotron radiation facilities, but at a lower cost and a smaller overall physical size.